The conventional analog television broadcasting uses a VHF band (88 MHz to 222 MHz). The ongoing transition from analog to digital broadcasting will cause a big change in band for use in television broadcasting.
That is, it has been decided that terrestrial digital broadcasting uses a UHF band (470 MHz to 710 MHz). After the end of analog broadcasting, the VHF band (88 MHz to 222 MHz) will be allotted to new broadcasting services.
Meanwhile, some small mobile terminals such as mobile phones have been prepared which can receive digital broadcasts such as digital radio broadcasts and digital television broadcasts, and such mobile terminals are becoming widespread. Further, there has been a tendency toward enrichment of broadcast content dedicated to mobile terminals such as one-segment mobile terminals. Therefore, mobile terminals are required to deal with a wide range of bands such as an FM radio band (75 MHz and a band located thereby), the VHF band, and the UHF band.
A conventional mobile terminal generally uses an earphone antenna as an antenna to receive such various broadcasts. The earphone antenna is used both as an earphone and an antenna. That is, the earphone antenna functions both as an earphone for outputting sounds and an antenna for receiving broadcast waves.
A typical earphone antenna includes a coaxial cable and an earphone cable. The coaxial cable includes a central conductor and an outer conductor that are insulated from each other. The earphone cable is a sound transmitting wire that serves also as a radiating element, and is connected to the coaxial cable. Generally, each of the coaxial cable and the earphone cable has a length of one-quarter resonant wavelength of an FM or VHF radio wave.
Moreover, when the coaxial cable and the earphone cable are fed with unbalanced power, the outer conductor of the coaxial cable and the earphone cable operate as a sleeve antenna suitable for reception of FM and VHF radio waves.
However, in cases where the length of each of the coaxial cable and the earphone cable is set to one-quarter resonant wavelength of a VHF broadcast wave, the coaxial cable and the earphone cable become much longer than the effective resonant length of a UHF broadcast wave. Therefore, the conventional earphone antenna has been low in reception sensitivity to UHF radio waves that are used for terrestrial digital broadcasting and the like.
In view of this, Patent Document 1 mentioned below discloses an earphone antenna having two earphone cables one of which has a length of one-quarter resonant wavelength of a UHF radio wave, thereby increasing reception sensitivity to UHF radio waves.
[Patent Document 1]
Japanese Unexamined Patent Application Publication No. 64742/2005 (Tokukai 2005-64742; published on Mar. 10, 2005)
However, even in cases where one of the earphone cables has a length of one-quarter resonant wavelength of a UHF radio wave, it is still difficult to obtain sufficient reception sensitivity.
This is, for example, because a typical coaxial cable has an outer conductor whose surface area is larger than the surface area of an earphone cable. That is, a leak current (unbalanced current) by which the large-surface-area outer conductor of the coaxial cable is excited becomes dominant over an electrical current flowing through the earphone cable.
With this, the influence of an electrical current flowing through the outer conductor of the coaxial cable which outer conductor has a length equal of one-quarter resonant wavelength of a VHF radio wave becomes greater than the influence of an electrical current flowing through the earphone cable that has a length of one-quarter resonant wavelength of a UHF radio wave.
Therefore, even in cases where the length of one of the earphone cables is set to one-quarter resonant wavelength of a UHF radio wave, the effect of setting the length of one of the earphone cables to one-quarter resonant wavelength of a UHF radio wave is cancelled by the influence of an electrical current flowing through the outer conductor of the coaxial cable. This makes it difficult to obtain sufficient sensitivity to broadcasts.
On the other hand, in cases where the length of each of the earphone cable and the coaxial cable is set to one-quarter wavelength of a UHF radio wave for the purpose of increasing reception sensitivity to UHF radio waves, the outer conductor of the coaxial cable and the earphone cable operate as a sleeve antenna suitable for reception of UHF radio waves. This makes it possible to increase reception sensitivity to UHF radio waves.
However, an earphone cable for use in the UHF band has a length as short as approximately a twentieth of one-quarter resonant wavelength of an FM or VHF radio wave. This undesirably causes remarkable deterioration in reception sensitivity in the FM and VHF bands.
Thus, there has conventionally been such a problem that it is impossible to realize an antenna that has good sensitivity in both the VHF and UHF bands.